1. Field of the Invention
This invention relates generally to postage mailing systems and more particularly to scales having piezoelectric transducers for generating weight information signals.
2. Brief Description of the Prior Art
Several problems have been encountered in the design of automated mail processing equipment wherein mail pieces to be posted were sorted, sealed, weighed and the appropriate postage affixed. Equipment designs incorporated various stations for mail processing with individual mail pieces being transported along a flow path between stations.
Prior weighing devices incorporated a deflectable weighing pan upon which the article was placed. Traditionally, the pan was spring biased and deflected under the weight of the article. Such deflection was measured and appropriate signal which was derived from the spring constant and the measured deflection was generated.
The deflection of scale pans has heretofore been detected by various optical systems as that disclosed in U.S. Pat. No. 4,044,847 issued to Wu on Aug. 30, 1977 and assigned to the assignee of the present invention, and U.S. Pat. No. 3,861,480 assigned to the assignee of the present invention. Unfortunately, the implementation of spring biased scales in an automated mailing system required significant dwell or weighing cycle time in order to permit the pan to deflect and the concommitant oscillations to dampen. Further, the use of damping mechanisms to retard oscillation merely increased the scale response time.
Because of the scale pan deflection, each mail piece had to be removed from the weighing station at a lower transport elevation than the entrance elevation. Such transport attitude changes detracted from high speed operation and efficient flow path designs.
Additional problems had been encountered with respect to prior transducers employed to detect scale deflection and provide appropriate weight indicative signals. Optical systems were expensive and their installation was time consuming and costly due to optical resolution requirements.
Prior approaches at providing weight indicative signals through the use of strain guages to measure the deflection of a spring biased scale required additional signal processing, initially amplifying the traditionally low output and then converting an amplified analog transducer signal into an appropriate digital signal for utilization in digital processing for computation of postage. It should be appreciated that the employment of amplifiers and analog-to-digital converters for signal processing introduced additional error sources in the mailing system.